Kinetic Study on Achiral-to-Chiral Transformation of Achiral Poly(diphenylacetylene)s via Thermal Annealing in Chiral Solvent: Molecular Design Guideline for Conformational Change toward Optically Dissymmetric Structures

Abstract

Achiral poly(diphenylacetylene)s (PDPAs: pMe3, pEt3, piPr3, pMe2O1, pMe2OD1, and mMe3) with different alkyl side chains at the para or meta position of the side phenyl ring were prepared to examine achiral-to-chiral transformations upon thermal annealing in a chiral solvent. All the para-substituted PDPAs showed significant circular dichroism (CD) enhancement upon annealing, whereas the meta-substituted polymer, mMe3, was inert to the same treatment. To investigate the kinetics, the asymmetric conformational change was monitored by observing the changes in the magnitude of circular polarization (g CD ) or optical rotation. PDPAs with bulkier, round-shaped side groups (pEt3 and piPr3) had greater g CD values at equilibrium than pMe3 with a smaller side group. Moreover, the activation energy for the forward reaction (Ea f ) was lower in the bulkier polymers than in pMe3 owing to enhanced miscibility with the chiral solvent. Similarly, the long alkyl chains of pMe2O1 and pMe2OD1 acted as internal plasticizers to lower their Ea f values relative to that of pMe3, whereas their g CD values at equilibrium were smaller than that of pMe3. The kinetics of the achiral-to-chiral transformation is discussed in detail based on the spectroscopic changes observed during the annealing process.

Dissymmetric Chiral Poly(diphenylacetylene)s: Secondary Structure Elucidation and Dynamic Luminescence

“…However, the presence of a phenyl group on each carbon of the double bond in PDPAs makes the properties of both materials very different. For instance, PDPAs show better chemical, thermal and photochemical stability compared to PPAs [72–85] . Moreover, PDPAs exhibit another interesting property that PPAs do not have, i.e.…”

“…Only a few examples related to chiral PDPAs [72–85] are found in the literature compared to the large amount of work that has been done with PPAs [12–33, 37–51, 94–97] . This is due to the difficulty of obtaining the former polymers in the laboratory by polymerization of the corresponding diphenylacetylene monomers (DPA).…”

“…photoluminescence [82–85, 89] . As a negative characteristic, PDPAs show a poor dynamic behavior, being necessary to heat them to 80° to induce a helical sense once the external stimuli are modified [79–85] . This depletion of dynamic behavior is a consequence of the introduction of the second phenyl ring in the m.r.u., which congests the helical backbone and where steric repulsion does not favor chiral amplification and helix inversion effects.…”

Solvent‐dependent helix inversion in optically active poly(diphenylacetylene)s and their chiral recognition abilities as chiral stationary phases for high‐performance liquid chromatography

“…25 These results suggest that the helix inversion energy of PDPAs is relatively high because of the steric repulsion between the pendant phenyl groups, and thermal annealing is necessary to induce a thermodynamically stable preferredhanded helical structure. 21,25,26 In addition, when these polymers were applied to CSPs for HPLC, (À)-h-poly-1S showed a higher chiral recognition ability toward many racemates than the as-prepared poly-1S with almost no helix-sense bias. Therefore, the one-handed helical structure induced in the polymer backbone plays an important role in the excellent chiral discrimination of PDPA-based CSPs.…”

Dissymmetric Chiral Poly(diphenylacetylene)s: Secondary Structure Elucidation and Dynamic Luminescence

“…By using a combination of 1 H and 13 C NMR, IR, Raman, VCD and ECD spectroscopies, they found that this polymer adopts a preferred cis‐transoidal structure, where P or M ‐helical senses can be induced by thermal annealing in water in the presence of chiral amines. On the other hand, in the case of asymmetric PDPAs, Kwak, Aoki and co‐workers made great contributions to the study of their luminescent properties and to the achiral‐to‐chiral transformations of PDPAs [78, 82, 85–89] . To perform these studies, a catalyst developed by Masuda [TaCl 5 ‐n‐Bu 4 Sn] was used [77] .…”